1. Technical Field
This disclosure generally relates to a driving circuit for supplying a current to a load, such as a light emission diode (LED) to drive the load, and an electronic device using such a driving circuit.
2. Description of the Related Art
A light emission element, such as a light emission diode (LED), is used as a back light of a liquid crystal display unit in a cellular phone.
FIG. 5 is a circuit diagram illustrating an example of a configuration of a driving circuit for driving a LED in the related art.
As showing in FIG. 5, a driving circuit 101 is used to drive a series of LEDs Le1 through Len (n is an integer greater than 1). The driving circuit 101 includes plural constant-current drivers Dri1 through Drin, which are connected to the LEDs Le1 through Len, respectively, a voltage comparator 102, and a DC-DC converter 103.
Each constant-current driver Drik (k=1, . . . , n) includes a constant current source CSok (k=1, . . . , n) and a current mirror circuit CMok (k=1, . . . , n).
The DC-DC converter 103 is connected to anodes of the LED series Le1 through Len, and supplies a voltage Vh0 to the serial circuits each including one of the LEDs Le1 through Len and an output transistor of one of the current mirror circuits CMo1 through CMon corresponding to the above one LED.
The voltage comparator 102 compares respective output voltages V1 through Vn of the current mirror circuits CMo1 through CMon, selects and outputs the lowest voltage Vlow. The DC-DC converter 103 adjusts the output voltage Vh0 so that the lowest voltage Vlow output from the voltage comparator 102 becomes equal to a reference voltage Vref, and thereby, maintaining the output voltage Vh0 to be constant.
Here, because fluctuations occur in characteristics of the LEDs constituting the LED series Le1 through Len, the output currents of the constant current source CSo1 through CSon, and characteristics of the current mirror circuit CMo1 through CMon, the voltages V1 through Vn also fluctuate. Therefore, in order that the constant-current drivers Dri1 through Drin carry out constant-current operations reliably, the reference voltage Vref is set while taking into consideration the maximum possible fluctuation of the LEDs, the constant current source CSo1 through CSon, and the current mirror circuit CMo1 through CMon.
In the driving circuit 101 as shown in FIG. 5, the output voltage Vh0 is adjusted so that the lowest voltage Vlow among the output voltages V1 through Vn of the current mirror circuits CMo1 through CMon becomes equal to the reference voltage Vref, and thereby, all of the LEDs can sufficiently emit light even though there exist fluctuations in characteristics of the LEDs.
In the related art, for example, Japanese Laid Open Patent Application No. 2003-332624 discloses a device for driving light emission elements in which the lowest voltage among the voltages applied to plural constant-current drivers connected to respective plural light emission element series is selected as a detection voltage, and the output voltage of a power supplier circuit is automatically adjusted so that the detection voltage becomes a sufficiently low voltage allowing the constant current operation of the constant current driver.
However, in the driving circuit of the related art, since the reference voltage Vref of the DC-DC converter 103 is set while taking into consideration the maximum possible fluctuations of the characteristics of the LEDs, the output currents of the constant current source CSo1 through CSon, and the characteristics of the current mirror circuit CMo1 through CMon, sometimes the output voltage Vh0 of the DC-DC converter 103 becomes unnecessarily high. Due to this, the electrical efficiency of the DC-DC converter 103 is degraded, and as a result, the power consumption of the total driving circuit becomes high.